Abstract: The invention relates to a process for preparing 1,2?-benzisothiazoline-3-one according to formula (I), comprising the following steps: (a) reacting a 2-halogenbenzonitrile compound of general formula (II) with a reaction mixture, containing: (i) alkaline sulphide and/or alkaline hydrogen sulphide and (ii) an alkyl halide compound, represented by general formula (III): R1X (III) for producing an intermediate product of general formula (IV), and (b) reacting the intermediate product of general formula (V) obtained in step (a) with a halogenating agent or an oxidant and subsequent reaction of the 2-(alkylsulfoxy)benzonitrile with an acid to form 1,2-benzisothiazoline-3-one as well as a halide compound of general formula (V) R1X (V).